Presently various fluids are used for heat transfer. The suitability of the heat-transfer fluid depends upon the application process. For example, some electronic applications require a heat-transfer fluid which is inert, has a high dielectric strength, has low toxicity, good environmental properties, and good heat transfer properties over a wide temperature range. Other applications require precise temperature control and thus the heat-transfer fluid is required to be a single phase over the entire process temperature range and the heat-transfer fluid properties are required to be predictable, i.e., the composition remains relatively constant so that the viscosity, boiling point, etc. can be predicted so that a precise temperature can be maintained and so that the equipment can be appropriately designed.
In the semiconductor industry, there are numerous devices or processes that require a heat-transfer fluid having select properties. The heat-transfer fluid may be used to remove heat, add heat, or maintain a temperature.
Each of the semiconductor processes described below incorporates a device or a work-piece which has heat removed from it or has heat added to it. The heat transfer associated with either the heat removal or addition can take place over a wide temperature range. Thus, in each case a heat-transfer fluid is preferably used which has other attributes that make it “operator friendly”. In order for a heat-transfer fluid to be considered “operator friendly”, the heat-transfer fluid preferably exhibits low toxicity and low flammability.
For automated test equipment (ATE), equipment is used to test the performance of semiconductor dice. The dice are the individual “chips” that are cut from a wafer of semiconductor substrate. The dice come from the semiconductor foundry and must be checked to ensure they meet functionality requirements and processor speed requirements. The test is used to sort “known good dice” (KGD) from dice that do not meet the performance requirements. This testing is generally performed at temperatures ranging from about −80° C. to about 100° C.
In some cases the dice are tested one-by-one, and an individual die is held in a chuck. This chuck provides, as part of its design, provision for cooling the die. In other cases, several dice are held in the chuck and are tested either sequentially or in parallel. In this situation, the chuck provides cooling for several dice during the test procedure.
It may also be advantageous to test dice at elevated temperatures to determine their performance characteristics under conditions of elevated temperature. In this case, a coolant which has good heat-transfer properties well above room temperature is advantageous.
In some cases, the dice are tested at very low temperatures. For example, Complementary Metal-Oxide Semiconductor (“CMOS”) devices in particular operate more quickly at lower temperatures.
If a piece of ATE equipment employs CMOS devices “on board” as part of its permanent logic hardware, it may be advantageous to maintain the logic hardware at a low temperature.
Therefore, to provide maximum versatility to the ATE, a heat-transfer fluid preferably performs well at both low and high temperatures (i.e., preferably has good heat transfer properties over a wide temperature range), is inert (i.e., is non-flammable, low in toxicity, non-chemically reactive), has high dielectric strength, has a low environmental impact, and has predictable heat-transfer properties over the entire operating temperature range.
Etchers operate over temperatures ranging from about 70° C. to about 150° C. In this process, reactive plasma is used to anisotropically etch the features in a wafer. The wafers to be processed are kept at a constant temperature at each selected temperature. Therefore, the heat-transfer fluid preferably is a single phase over the entire temperature range. Additionally, the heat-transfer fluid preferably has predictable performance over the entire range so that the temperature can be precisely maintained.
Ashers operate over temperatures ranging from about 40° C. to about 150° C. This is a process that removes the photosensitive organic “mask”.
Steppers operate over temperatures ranging from about 40° C. to about 80° C. This is the process step in semiconductor manufacturing where the reticules needed for manufacturing are produced. Reticules are used to produce the patterns of light and shadow needed to expose the photosensitive mask. The film used in the steppers is typically maintained within a temperature window of +/−0.2° C. to maintain good performance of the finished reticule.
PECVD (plasma enhanced chemical vapor deposition) chambers operate over temperatures ranging from about 50° C. to about 150° C. In this process, films of silicon oxide, silicon nitride, and silicon carbide are grown on a wafer by the chemical reaction initiated in a reagent gas mixture containing silicon and either: 1) oxygen; 2) nitrogen; or 3) carbon. The chuck on which the wafer rests is kept at a uniform, constant temperature at each selected temperature.
Heat-transfer fluids which are presently used in these semiconductor applications include perfluorocarbons (PFCs), perfluoropolyethers (PFPEs), perfluoroamines (PFAs), perfluoroethers (PFEs), water/glycol mixtures, deionized water, silicone oils and hydrocarbon oils. However, each of these heat-transfer fluids has some disadvantage. PFCs, PFPEs, PFAs and PFEs may exhibit atmospheric lifetime values of greater than 500 years, and up to 5,000 years. Additionally, these materials may exhibit high global warming potentials (“GWP”). GWP is the integrated potential warming due to the release of one (1) kilogram of sample compound relative to the warming due to one (1) kilogram of CO2 over a specified integration time horizon. Water/glycol mixtures are temperature limited, that is, a typical low temperature limit of such mixtures is −40° C. At low temperatures water/glycol mixtures also exhibit relatively high viscosity. The high viscosity at low temperature yields high pumping power. Deionized water has a low temperature limit of 0° C. Silicone oils and hydrocarbon oils are typically flammable.
Removing heat from electronic devices has become one of the most important obstacles to further improving processor performance. As these devices become more powerful, the amount of heat generated per unit time increases. Therefore, the mechanism of heat transfer plays an important role in processor performance. The heat-transfer fluid preferably has good heat transfer performance, good electrical compatibility (even if used in “indirect contact” applications such as those employing cold plates), as well as low toxicity, low (or non-) flammability and low environmental impact. Good electrical compatibility requires the heat-transfer fluid candidate to exhibit high dielectric strength, high volume resistivity, and poor solvency for polar materials. Additionally, the heat-transfer fluid candidate must exhibit good mechanical compatibility, that is, it must not affect typical materials of construction in an adverse manner. In this application, heat-transfer fluid candidates are disqualified if their physical properties are not stable over time.
Materials currently used as heat-transfer fluids for cooling electronics or electrical equipment include PFCs, PFPEs, silicone oils, and hydrocarbon oils. Each of these heat-transfer fluids has some disadvantage. PFCs and PFPEs may be environmentally persistent. Silicone oils and hydrocarbon oils are typically flammable.
Thermal shock testing is generally performed at temperatures ranging from about −65° C. to about 150° C. The rapid cycling of temperature in a part or device may be required to simulate the thermal changes brought on by, for instance, launching a missile. Thermal shock testing is required for electronics used for military missiles, among other things. There are several military specifications related to thermal shock testing of many electronic components and assemblies. This test uses various means of imparting rapidly changing temperatures within a part or electronic device. One such device employs a liquid heat-transfer fluid or liquid heat-transfer fluids that are kept in separate reservoirs maintained at temperature extremes where parts are alternately immersed to induce thermal shock to the test part. Typically, operators load and unload the components or assemblies to and from the thermal shock equipment. Therefore, it is important that a heat-transfer fluid used in such an application exhibit low toxicity, low flammability, and low environmental impact. Heat-transfer fluids which are liquid over a wide temperature range coupled with low toxicity, low flammability, and low environmental impact are ideal for thermal shock testing.
Materials currently used as heat-transfer fluids for liquid/liquid thermal shock test baths include liquid nitrogen, PFCs, and PFPEs. Each of these heat-transfer fluids has some disadvantage. Liquid nitrogen systems offer limited temperature selectivity at the low temperature end. PFCs and PFPEs may be environmentally persistent.
Constant temperature baths are typically operated over a broad temperature range. Therefore, desirable heat-transfer fluids preferably have a wide liquid range and good low-temperature heat transfer characteristics. A heat-transfer fluid having such properties allows a very wide operating range for the constant temperature bath. Typically, most testing fluids require fluid change-out for wide temperature extremes. Also, good temperature control is essential for accurately predicting physical properties of the heat-transfer fluids.
Heat-transfer fluids which are presently used in this application include: PFCs, perfluoropolyethers (PFPEs), water/glycol mixtures, deionized water, silicone oils, hydrocarbon oils, and hydrocarbon alcohols. Each of these heat-transfer fluids has some disadvantage. PFCs and PFPEs may be environmentally persistent. Water/glycol mixtures are temperature limited, that is, a typical low temperature limit of such mixtures is −40° C. At low temperatures water/glycol mixtures also exhibit relatively high viscosity. Deionized water has a low temperature limit of 0° C. Silicone oils, hydrocarbon oils and hydrocarbon alcohols are typically flammable.
For heat-transfer processing requiring an inert fluid, fluorinated materials are often used. Fluorinated materials typically have low toxicity, are essentially non-irritating to the skin, are non-chemically reactive, are non-flammable, and have high dielectric strength. Fluorinated materials such as perfluorocarbons, perfluoropolyethers, and hydrofluoroethers provide the additional advantage of not depleting the ozone layer in the stratosphere.
As discussed above, perfluorocarbons, perfluoropolyethers, and some hydrofluoroethers have been used for heat-transfer.
Perfluorocarbons (PFCs) exhibit several traits advantageous to the applications discussed above. PFCs have high dielectric strength and high volume resistivity. PFCs are non-flammable and are generally mechanically compatible with materials of construction, exhibiting limited solvency. Additionally, PFCs generally exhibit low toxicity and good operator friendliness. PFCs are manufactured in such a way as to yield a product that has a narrow molecular weight distribution. They do exhibit one important disadvantage, however, and that is long environmental persistence.
Perfluoropolyethers (PFPEs) exhibit many of the same advantageous attributes described for PFCs. They also have the same major disadvantage, i.e., long environmental persistence. In addition, the methods developed for manufacturing these materials yield products that are not of consistent molecular weight and thus are subject to performance variability.
Hydrofluoropolyethers (HFPEs), a class of hydrofluoroethers (HFEs), exhibit some of the same advantageous attributes of PFCs, but differ greatly in two areas. To their credit, they exhibit markedly lower environmental persistence, yielding atmospheric lifetimes on the order of decades rather than millennia. However, some of the HFPEs taught as heat-transfer fluids are a mixture of components of widely disparate molecular weight. Thus, their physical properties may change over time which makes it difficult to predict performance.
Some hydrofluoroethers have been disclosed as heat-transfer fluids. However, the need exists for a heat-transfer fluid which is inert, has high dielectric strength, low electrical conductivity, chemical inertness, thermal stability and effective heat transfer, is liquid over a wide temperature range, has good heat-transfer properties over a wide range of temperatures and also has a shorter atmospheric lifetime, and therefore a lower global warming potential, than existing heat-transfer fluids.